A wireless communication system comprises an access network with radio network controllers (RNCs) and radio base stations (RBS) consisting of base band units (BBUs) performing base band signal processing. The RBS consist further of one or more radio remote units (RRUs), which convert the transmitting and receiving signals between baseband and radio frequencies and transmits and receives signals over one or more antennas. The base station architecture supports the connection between one baseband unit and several distributed RRUs.
Some wireless communication systems e.g. WCDMA and LTE systems support the application of multiple antenna techniques, e.g. MIMO techniques and beam forming techniques. A base station system using an adaptive beam forming algorithm is expected to increase its maximum range and capacity due to an improved antenna gain and the reduction of interference from other spatially separated users. E.g. an antenna array containing M elements can provide a mean power gain of M over white noise. Beam forming algorithms often assume that the antenna array has no errors and that the multi-channel transceiver has an identical transfer function. However due to mechanical and electrical variations in the RF components such as the amplifiers, mixers and cables that occur over time because of temperature variations, aging, and other factors, the spatial signature of the baseband receive/transmit signal is significantly different from that of the RF receive/transmit signal. This leads to the fact that the transfer functions of the RF transceivers are different from each other, i.e. the amplitude, time and phase deviations of the signals received/transmitted on the different antenna elements vary. In order to achieve the antenna gain it is important to perform antenna calibration compensating the amplitude, time and phase deviation among the different antenna elements. Especially for a LTE-TDD (Long Term Evolution—Time Division Duplex) system the antenna calibration across the antenna calibration chain i.e. the base station (i.e. eNodeB) transmit/receive chain is important when exploiting reciprocity.
When pursuing antenna processing solutions e.g. beamforming of the transmitted and received signals in a distributed RRU system, calibration of the amplitude, time and phase deviation among the different antenna elements in the different RRUs is performed. However, the antenna calibration among the different antenna elements in the different RRUs may have a strict limitation e.g. in a LTE TDD system calibration duration may be ask for in a special slot. The slot should be far less than an OFDM symbol duration, which symbol duration equals to about 71 μs. However, the receiving of the calibrated signal maybe out of the required range of the time window if the RRU is located far away.
The distance D between the BBU and the RRU result in a transmission delay described asΔt=2D/3*108 
Provided that the calibration duration i.e. the time acquired to compensate the transmission delay should be less than 20 μs, the cable distance between the BBU and the farthest RRU should be less than 3 km. When presuming additional delay introduced by the electronic components in the RRU and the BBU such as processing delay, it could be assumed that the distance between the BBU and the farthest RRU should be even less than 3 km. This may limit the deployment of the distributed RRU system.